Introduction/Background Evaluate benefit with interstitial (IC/IS) IGBT in terms on dose on target volume while managing LACC, after external RCT, compared to treatment with only intracavitary (IC) IGBT.

Methodology All patients were treated with IC/IS IGBT after RCT. IC/IS IGBT was compared to IC IGBT for target volume cover (GTV, HR-CTV, IR-CTV). We evaluated overall survival, local control and toxicity with IC/IS IGBT. Local control was analysed by years of treatment to assess improvement over time.

Results From 01/2017 to 12/2020, 99 patients (p) were analysed. FIGO 2009 classification: IIA 6p, IIB 45p, IIIB 22p, IVA 20p, IVB 6p. FIGO 2018 founded 24p IIIC1 and 28p IIIC2. Mean High Risk Clinical Target Volume (HRCTV) was 40 cm3 (9,6–103) with 66 (66,7%) patients presented a volume >30cc. The median Overall treatment Time (OTT) was 55 days (50 – 62). The mean D90 HR-CTV was 80,3Gy for patients treated by IC/IS, and 75,1Gy for IC (p<0.0001). A decrease of the delivered dose for all Organs at Risk (OAR) was found: D_{2 Bladder} less than 80Gy to IC/IS in 66,7% of patients and 27,3% of patients without IS (p<0,0001); D_{2 Rectum} is less 65Gy in 32,3% of patients with IC/IS and 17,2% of patients without (p<0,001); and D_{2 Sigmoid} is less than 70Gy in 99% of patients with IC/IS and in 94,9% of patients without (p<0,05). The overall survival (OS) was 66,2% at 2 years, and local control (LC) was 56,5% at 2 years. Local survival improved over time, with better control in 2020 (p=0,036).

Conclusion Dose to HRCTV is higher with IC/IS IGBT compared to IC IGBT with lower doses to OAR in patients managed for LACC after RCT